"I`m tempted to believe that the fourth tunnel is just a bluff to put pressure on the international community to resume discussion with them," Lassina Zerbo, head of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), told Reuters.
North Korea has conducted three underground nuclear tests since 2006.
The last, in 2013, drew international condemnation including from China, its main diplomatic ally. Isolated North Korea is under U.N. sanctions that prohibit trade that can fund its arms program.
Diplomatic talks amongst six countries including the United States and North Korea aimed at ending its nuclear program produced a deal in 2005 from which the North has since walked away.
North Korea has been steadily working on its nuclear program, but a fourth test is not see as imminent.
While a new tunnel at the site could well indicate North Korea was preparing a test, the timing would "probably depend on political factors more than technical ones", said Jeffrey Lewis, author of the 38 North report.
The North`s agreement with South Korea in August to work towards easing tension and improving ties also meant a test was less likely, said Zerbo.
"One would not anticipate that North Korea would go for new testing at a time when they are engaged in discussions with their brothers from the South," Zerbo said.
"In the 21st century, they are the only country to have conducted a nuclear test explosion. In the 21st century, the world is too civilized for anyone to resume testing".
More than 160 countries have ratified the Comprehensive Nuclear-Test-Ban Treaty since 1996.
India and Pakistan have also conducted nuclear tests since then and are among eight countries including the United States and China preventing the treaty`s entry into force.
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